High temperature sensors such as pressure sensors, gas sensors, accelerometers, acoustic sensors, etc., are critical for advanced industrial, automotive and aerospace sensing applications with typical temperatures for these applications ranging from 200 to 1000° C., and above. As such, the need for electronic systems, including sensors to monitor noise, vibration, and acoustic emissions at high temperatures is known. The need for actuators, such as linear positioning systems, nano-positioning systems, etc., that can operate at high temperatures is also known.
Sensing can be accomplished by a wide variety of commercially available devices, including: piezoelectric sensors; strain gauges; piezoresistive sensors, capacitive sensors, to name a few. Of the several sensing mechanisms available, piezoelectrics are the most prevalent due to their simplicity of design, integration and high sensitivity over a broad range of frequencies and temperature.
It is appreciated that piezoelectric sensors not only develop a charge for an applied stress or strain, but also maintain such a charge for a period of time long enough to be detected by the electronic system. It is also appreciated that the length of time the charge is maintained is proportional to the RC time constant. Furthermore, the minimum useful frequency of a sensor, known as the lower limiting frequency (fLL), is inversely proportional to the RC time constant and below fLL the charge will drain off before it can be detected as a result of conduction in the sensor. Finally, with a low fLL, the dynamic bandwidth can be extended into the audio frequency range; thus, a large RC constant (especially, high electrical resistivity at an elevated temperature) is desirable.
A broad range of ferroelectric and nonferroelectric piezoelectric crystals for sensors have been investigated. For example, FIG. 1 summarizes sensitivity versus maximum usage temperature for various prior art piezoelectric materials based, which are briefly discussed below. In addition, the maximum usage temperature shown in FIG. 1 is based on a material's Curie temperature, melting temperature, phase transition temperature and/or proposed electrical resistivity of 106 Ohm·cm, though materials with lower electrical resistivity may still be functional in different applications, for example with high frequency sensing applications.
The relaxor-PbTiO3 ferroelectric single crystals (labeled Perovskite Crystals) have a perovskite structure and are found to possess the highest piezoelectric properties with d33 and d15 values on the order of >2000 pC/N. However, their usage temperature range is limited by low ferroelectric phase transitions TRT (<120° C.) with a correspondingly strong temperature dependence.
Perovskite polycrystalline ceramics (labeled Perovskite Ceramics), such as Pb(Mg1/3Nb)O3—PbTiO3 (PMNT), Pb(Zr,Ti)O3 (PZT) and BiScO3—PbTiO3 (BSPT), have sensitivities in the range of 100 pC/N to 1000 pC/N, with a usage temperature range of 80-350° C. The usage temperature range is restricted by thermally activated aging at temperatures far below their Curie temperatures, above which they suffer depolarization and complete loss of piezoelectric activity.
Ferroelectric materials with the tungsten bronze structure (labeled Tungsten Bronze) and Aurivillius structure (labeled BLSF) possess modest piezoelectric properties. For example, sensitivities range from 10 pC/N to 100 pC/N for usage temperatures up to 600° C., which is limited by their respective Curie temperatures and/or low electrical resistivities. It should be noted that though lithium niobate (labeled LN) crystals possess a Curie temperature of 1150° C., their low resistivities and oxygen loss at elevated temperatures restrict applications to less than 600° C., however operational temperature ranges can be much wider for high frequency usage.
In general, nonferroelectric piezoelectric single crystals possess low sensitivities, e.g. falling in the range of 1-20 pC/N (see Table 1 below). However, their ultralow mechanical and dielectric losses, and high electrical resistivities, make them ideal candidates for high temperature sensing applications. Also, the usage temperature range of piezoelectric crystals may be limited by α-β phase transitions, the melting point and/or low electrical resistivity at ultrahigh temperature. In addition, the low symmetry results in undesirable cross-talk and pyroelectric effects that may dominate the response signal.
TABLE 1deffρ(Ω · cm)MaterialsGrowth methodCostsymmetry(pC/N)keff (%)ϵr@600° C.CommentsTourmalineMineralLow3 m1.8-3.69-117.5-8.2108MineralAlNsublimation/6 mm5.6/~12/Oxidizationα-SiO2hydrothermalLow322.3 84.5/α-β transitionGaPO4hydrothermalHigh324.5166.1109α-β transitionLGSCz/BridgmanHigh326.21619.2106Disorder structureLGTCz/BridgmanHigh326.41619.6>106  Disorder structureCTGSCz/BridgmanHigh324.61218.2108Low piezoelectricYCOBCz/BridgmanLowm 3-10 6-2212.0>109  Low symmetryNdCOBCz/BridgmanLowm11-1619-3155.5108Low symmetryLGS: La3Ga5SiO14 with disordered structure;LGT: La3Ga5.5Ta0.5O14 with disordered structure;CTGS: Ca3TaGa3Si2O14 with ordered structure.
Quartz α-SiO2 is the best known piezoelectric material and is widely used in electronic devices, such as oscillators, resonators and filters. Originally, natural quartz crystals were employed, but now have been widely replaced by hydrothermally grown synthetic quartz. Quartz possesses excellent electrical resistivity (>1017 Ω·cm at room temperature) and ultralow mechanical loss (high mechanical QM), narrow bandwidth and temperature-stability, thus making it the material of choice in telecommunication equipment. However, disadvantages of quartz include relatively small piezoelectric coefficients (d11˜2.3p C/N) and a low α-β phase transition temperature at 573° C., which is further limited by mechanical (ferrobielastic) twinning that occurs at 300° C. Tourmaline is a natural mineral with a complex aluminum-borosilicate composition and a large variation in electrical resistivity. The piezoelectric coefficient d33 is reported to be 1.8 pC/N. Tourmalines have the advantage over quartz of no twining or phase change prior to their respective melting points, and have been commercialized for pressure or vibration sensors at temperatures less than or equal to 6000° C. However, tourmaline belongs to 3 m symmetry, which shows strong pyroelectric effects. In addition, with tourmaline being a natural mineral, the quality is depended on various sources and attempts to artificially grow tourmaline crystals with usable size have proven unsuccessful.
Gallium orthophosphate (GaPO4) is a quartz analogue that belongs to the 32 symmetry and shows no pyroelectric effect. Gallium orthophosphate shares many of the positive features of quartz, such as high electrical resistivity, high mechanical quality factor and temperature stability up to 970° C. where an α-β phase transition occurs. However, the piezoelectric coefficient is only 4.5 pC/N and the material is further limited by the high production costs.
The langasite family of crystals belongs to the trigonal system (point group 32), is not pyroelectric and have a general formula of A3BC3D2O14. Langasite—La3Ga5SiO14 (LGS), and its isomorphs, such as langatate—La3Ga5.5Ta0.5O14 (LGT), can be readily grown and have been widely commercialized for pressure sensors and accelerometers with moderate piezoelectric coefficients of 6-7 pC/N. However, their usage temperature is less than or equal to 600° C. due to their crystal structures being disordered, which results in incoherent phonon scattering, increased the acoustic friction and greatly decreased electrical resistivity. On the other hand, the langasite crystals with ordered structure, such as Ca3TaGa3Si2O14, show much higher resistivity, but with relatively low piezoelectric coefficient of 4.6 pC/N.
Oxyborate crystals, with the general formula ReCa4O(BO3)3 (Re=rare earth element, abbreviated ReCOB) can be readily grown from the melt using the Czochralski (CZ) at around 1500° C. Analogous to langasite crystals, their potential temperature usage range is expanded due to no phase transition(s) occurring prior to their melting points. As such, oxyborate crystals can possess ultrahigh electrical resistivity at elevated temperatures, and high temperature stability of piezoelectric and electromechanical properties. For example, piezoelectric coefficients can be on the order of 3-16 pC/N, depending on the crystal orientations (cuts) and vibration modes. However, due to the low monoclinic symmetry (point group m), optimized crystal cuts to minimize the cross-talk and pyroelectric effect is problematic.
Given the above, it is apparent that improved sensors for use at high temperatures are needed. Therefore, high temperature sensors using a piezoelectric material that can be used at high temperatures, exhibit minimum cross-talk, and have low manufacturing costs would be desirable.